Separator, reactor, and method for producing aromatic carboxylic acids

ABSTRACT

In a process for producing an aromatic carboxylic acid, it is a dehydrating process which can achieve compactification of a step for removing water from a mixture of acetic acid and water produced in the production process, and which can reduce the consumed energy. In a production process of an aromatic carboxylic acid having an oxidation reaction step for producing a slurry of an aromatic carboxylic acid by carrying out liquid-phase oxidation reaction of an alkyl aromatic compound with an oxygen-containing gas in a solvent containing acetic acid in the presence of an oxidation catalyst, at least a portion of a mixture containing acetic acid and water produced in the production steps is separated into a permeable gas mainly comprising water and nonpermeable substances mainly comprising acetic acid, using a separation membrane having water selectivity.

TECHNICAL FIELD

The present invention relates to a method for producing an aromaticcarboxylic acid by subjecting alkyl substituents or a partially oxidizedaromatic alkyl compound to liquid-phase oxidation using anoxygen-containing gas, and a reactor used in this method.

BACKGROUND ART

Aromatic carboxylic acids are usually produced in an oxidation reactorby subjecting alkyl aromatic compounds as raw materials to liquid-phaseoxidation in a solvent containing acetic acid in the presence of acatalyst containing a heavy metal compound and a bromine compound by useof a gas containing oxygen in the form of molecules. During thisoxidizing reaction, water is produced. Reaction vapor discharged fromthe oxidation reactor and a mother liquid of aromatic carboxylic acidslurry contain reaction water. Such reaction vapor and mother liquid,the latter being obtained by separating aromatic carboxylic acids fromthe slurry produced, are typically dehydrated and distilled to recoveracetic acid, which is recycled as a solvent for oxidation reaction.

Since water and acetic acid are difficult to separate from each otherand because a dilute aqueous solution of acetic acid in particular has aspecific volatility near 1, in order to separate acetic acid from waterby dehydration and distillation so that the concentration of acetic acidin the distilled water will not exceed 1 weight percent, it is necessaryto increase the number of stages of the distilling column and/orincrease the reflux ratio. This increases the size of thedehydration/distillation column, which in turn pushes up its cost aswell as the facility cost. Also, because the latent heat of vaporizationof water is high, reboiler loads of distillation increases by increasingthe reflux ratio. Various measures have been reported for avoiding thisproblem. For example, patent document 1 reports a method of reducingdistillation loads by combining distillation with extraction. Patentdocument 2 reports that it is possible to reduce the reflux ratio andenergy consumption by azeotropic distillation using an azeotropic agentin the dehydration/distillation column.

Also known are methods of dehydration in which a separation membranesuch as a reverse osmosis membrane is used in distillation (patentdocuments 3 and 4). Patent document 3 proposes to remove water andalcohol by separation using a membrane in producing an aromaticcarboxylic acid. With this arrangement, in which distillation iscombined with separation using a membrane, it is possible to reduceloads on the dehydration/distillation column, so that an aliphaticcarboxylic acid, a solvent, can be recovered with high efficiency.

The alcohol mentioned in patent document 5 is an unnecessary byproductproduced when recovering acetic acid by hydrolyzing an aliphaticcarboxylic acid ester such as methyl acetate, which is a byproductproduced in the system. It is separated together with water by amembrane.

Patent document 1: JP patent publication 7-53443

Patent document 2: WO96-06065

Patent document 3: JP patent publication 2001-328957

Patent document 4: WO02-50012

Patent document 5: JP patent publication 2001-328957

DISCLOSURE OF THE INVENTION

As is apparent from patent documents 1 and 2, extraction agents andazeotropic agents are needed in the extraction and azeotropic methods,respectively. This complicates the dehydration/distillation steps. Themethods disclosed in these references are therefore not sufficientlystreamlined.

In an arrangement in which a separator membrane is used in distillation,such as disclosed in patent documents 3 and 4, it is an essentialrequirement to use a reverse osmosis membrane made of an organicmaterial in the separation step. But organic high-molecular membranes,which are typically used as acid-resistant separator membranes that canselectively separate water from an aqueous solution containing organicacids, have a drawback in that their heat resistance is so poor thatthey can be used only at relatively low temperatures.

Further, because it is an effective means for minimizing the loss of anacetic acid solvent to reuse methyl acetate in the oxidation reactionstep, the consumption of acetic acid will increase if alcohol isdischarged out of the system by hydrolyzing an aliphatic carboxylic acidester, as disclosed in patent document 5.

Still further, in this prior art, since any components that have notpermeated the membrane is returned into the distillation column,components that have permeated the membrane are diluted at the top ofthe distillation column. This increases the amount of vapor and theamount of fluid flow in the distillation column, which in turn makes itnecessary to increase the size of the distillation column and the areaof the separator membrane. Also, it is necessary to re-heat thecomponents that have not permeated the membrane and have been returnedinto the column. This requires additional energy consumption.

A general object of the invention is therefore to avoid all of theabovementioned problems, and its particular object is to provide aprocess for producing an aromatic carboxylic acid in which the step ofremoving water from a mixture of acetic acid and water that is producedduring the process can be carried out using a facility of a reduced sizewith reduced energy consumption.

The present inventors sought ways to achieve this object and found outthat by using a specific separator membrane, a mixture of acetic acidand water produced during a manufacturing process of an aromaticcarboxylic acid can be efficiently separated into water and acetic acid,using a compact device with minimum energy consumption.

Specifically, the invention provides a separation system comprising:

a distillation column into which a mixture of a first component mainlycomprising water and a second component mainly comprising nonaqueoussubstances is adapted to be supplied; a separator including a separationmembrane for separating overhead vapor discharged from a top of thedistillation column into a permeable vapor which mainly comprises thefirst component and a nonpermeable vapor which mainly comprises thesecond component by allowing only a selected portion of the overheadvapor to permeate the separation membrane; and a reflux unit for coolinga portion of the overhead vapor into a liquid and returning the liquidthus obtained into an upper portion of the distillation column.

With this arrangement, by returning a portion of the overhead vapor intothe distillation column through the reflux unit, it is possible toreduce the concentration of a high-boiling-point component in theoverhead vapor (either the first or second component). Thus, theseparation membrane has only to separate overhead vapor of which theconcentration of a high-boiling-point component has decreased. Thismakes it possible to reduce the concentration of high-boiling-pointcomponents in the vapor that has permeated the separation membrane to arequired level.

In the second invention, the distillation column includes fluid beds.

In the third invention, there is provided a separation systemcomprising: a distillation column into which a mixture of a firstcomponent mainly comprising water and a second component mainlycomprising nonaqueous substances is adapted to be supplied; a firstseparator including a first separation membrane for separating overheadvapor discharged from a top of the distillation column into a firstpermeable vapor which mainly comprises the first component and a firstnonpermeable vapor which mainly comprises the second component byallowing only a selected portion of the overhead vapor to permeate thefirst separation membrane; and a second separator including a secondseparation membrane for separating the first permeable vapor into asecond permeable vapor mainly comprising the first component and higherin the concentration of the first component than the first permeablevapor, and a second nonpermeable vapor which mainly comprises the secondcomponent by allowing only a selected portion of the first permeablevapor to permeate the second separation membrane.

After the overhead vapor has been separated into the first permeablevapor and the first nonpermeable vapor, the first permeable vapor isfurther separated into the second permeable vapor and the secondnonpermeable vapor. Thus, most of the portion of the second componentthat may permeate the first separation membrane will be separated as thesecond nonpermeable vapor by the second separation membrane. Thus, it ispossible to obtain a permeable condensed vapor high in the concentrationof the first component as the second nonpermeable vapor.

According to the fourth invention, there is provided a reactor systemcomprising: a reactor for producing an aromatic carboxylic acid andwater from an alkyl aromatic compound in a solvent containing aceticacid, and for generating a vapor mixture of a solvent and water; a firstseparation membrane for separating the vapor mixture, which isdischarged from the reactor, into a first permeable vapor mainlycomprising a first component and a first nonpermeable vapor mainlycomprising a second component; a second separation membrane forseparating the first permeable vapor, which is discharged from the firstseparation membrane, into a second permeable vapor mainly comprising thefirst component and a second nonpermeable vapor mainly comprising thesecond component; and a return passage for condensing and returning thefirst nonpermeable vapor and the second nonpermeable vapor into thereactor.

Any nonaqueous components that remain in the first permeable vaporwithout being separated by the first separation membrane are separatedby the second separation membrane, recovered as the second nonpermeablevapor, and returned into the reactor. Thus, it is possible to reduce thewater concentration in the reactor to less than a predetermined level,thereby accelerating reaction.

According to the fifth invention, solvent containing acetic acid isacetic acid, the alkyl aromatic compound is paraxylene, and the aromaticcarboxylic acid is terephtahlic acid.

The reactor according to the sixth invention further comprisesgas-liquid separators each provided between one of the first and secondseparation membranes and the return passage for separating terephthalicacid from the first and second nonpermeable vapors.

According to the seventh invention, the separation membrane or the firstand second separation membranes comprise an inorganic porous membercarrying in pores thereof a silica gel obtained by hydrolyzing analkoxysilane containing ethoxy groups or methoxy groups.

According to the eighth invention, there is provided a method ofproducing an aromatic carboxylic acid comprising an oxidation reactionstep in which an alkyl aromatic compound is reacted with aoxygen-containing gas in a solvent containing acetic acid in thepresence of an oxidation catalyst to produce a slurry of the aromaticcarboxylic acid; a solid-liquid separation step in which the slurry isseparated into a reaction mother liquid and an aromatic carboxylic acidcake; and a step of separating at least a portion of a mixture of aceticacid and water produced during the steps into a permeable gas mainlycomprising water and nonpermeable substances mainly comprising aceticacid, using a separation membrane capable of separating water.

In the step for producing an aromatic carboxylic acid using acetic acidas a solvent, water is produced as a byproduct during oxidationreaction. In distilling and separating acetic acid and water, since theevaporative latent heat of water is high, huge energy is needed. Byseparating the mixture into a permeable gas mainly comprising water anda nonpermeable substances mainly comprising acetic acid, using amembrane capable of separating water, i.e. a membrane that passesgaseous H₂O molecules but is less likely to pass nonpermeable substancesmainly comprising acetic acid, it is possible to reduce the energyneeded to separate acetic acid from water.

According to the ninth invention, in the arrangement of the firstinvention, at least a portion of the mixture fed to the separationmembrane is a gas. Since the substance that has permeated the separationmembrane according to the invention is a gas, if the mixture is also agas, it permeates the separation membrane more efficiently. By keepingthe temperature of the mixture higher than the boiling point of aceticacid at the operation pressure when fed to the separation membrane,substantially the entire mixture can be supplied in the form of a gas.Thus, it is possible to separate a greater amount of mixture in ashorter period of time.

According to the tenth invention, the mixture of acetic acid and watercontains methyl acetate, and at least a portion of the mixture isseparated into a permeable gas mainly comprising water and nonpermeablesubstances mainly comprising acetic acid and methyl acetate. Since aseparation membrane which can separate water is less likely to passmethyl acetate, it is present in the nonpermeable substances whichmainly comprise acetic acid. Methyl acetate can thus be recoveredtogether with acetic acid. This reduces the energy necessary to separatea mixture of acetic acid, methyl acetate and water into water and amixture of acetic acid and methyl acetate.

According to the 11^(th) invention, in the third invention, thenonpermeable substances are at least partially returned to the oxidationreaction step. The nonpermeable substances, i.e. the substances thathave not permeated the separation membrane, mainly comprise acetic acidand further contain methyl acetate. They scarcely contain water. On theother hand, oxidation reaction needs acetic acid as a solvent. Thus, byreturning the nonpermeable substances as a solvent for oxidationreaction, acetic acid contained in the nonpermeable substances can beeffectively used. Also, by recovering the methyl acetate contained inthe nonpermeable substances, which is one of the byproducts of theoxidation reaction, in the oxidation step, it is possible to suppressthe production of methyl acetate due to the equilibrium reaction ofacetic acid, thereby reducing the loss of the solvent.

According to the 12^(th) invention, in the third invention, beforemembrane separation, the mixture is supplied into the distillationcolumn, and at least part of acetic acid is recovered form the bottom ofthe column, and at least part of the mixture of acetic acid, methylacetate and water which is discharged from the top of the column issupplied to the separation membrane, which is capable of separatingwater.

The larger the carboxylic acid production plant, the greater the amountof a mixture that has to be separated. In such a case, the mixtureshould be fed into a small distillation column beforehand to produce anoverhead component having a reduced acetic acid content. By separatingsuch an overhead component with a separation membrane, it is possible toreduce the energy required for separation.

According to the 13^(th) invention, in the fifth invention, part of themixture discharged from the top of the column is returned into thedistillation column, and part of it is supplied to the separationmembrane.

By returning part of the overhead component, its acetic acid contentfurther decreases. Thus, by separating it through the separationmembrane, it is possible to further reduce the energy for separation.

According to the 14^(th) invention, in the fifth or sixth invention, thenonpermeable substances are returned to the oxidation reaction step.Thus, the methyl acetate contained in the nonpermeable substances can berecovered in the oxidation reaction step. This suppresses the productionof methyl acetate during the equilibrium reaction of acetic acid,thereby reducing the loss of the solvent.

According to the 15^(th) invention, the permeable gas mainly comprisingwater is further separated into a permeable gas mainly comprising waterand nonpermeable substances mainly comprising acetic acid, using aseparation membrane capable of separating water.

By providing the two separation membranes, the second separationmembrane removes any organic components that have not been removed bythe first separation membrane. High-purity water is thus obtained.

According to the 16^(th) invention, in the eighth invention, one of theseparation membranes that is provided upstream from the other is onethat is higher in the permeating speed, and the other is one that ishigher in the separation ability. With this arrangement, it is possibleto separate the mixture of water and acetic acid in large amounts andwith high purity.

According to the 17^(th) invention, in the first invention, theseparation membrane is made of an inorganic material. Since the membraneis formed of an inorganic material, it is durable and high in theability to separate water.

According to the 18^(th) invention, in the ninth invention, theseparation membrane or the separation membranes comprise an inorganicporous member carrying in pores thereof a silica gel obtained byhydrolyzing an alkoxysilane containing ethoxy groups or methoxy groups.

By using silica gel, it is possible to obtain water higher in purity.

According to the 19^(th) invention, the alkyl aromatic compound isparaxylene, and the aromatic carboxylic acid is terephthalic acid.Today, among aromatic carboxylic acids, terephthalic acid is beingproduced in the greatest amount. Thus, plants for manufacturingterephthalic acid are increasing in size year after year. The presentinvention is most advantageously applicable to such plants.

According to the present invention, it is possible to reduce the size ofsystems for recovering solvents such as distillation columns and toreduce the energy consumed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a separation system of a firstembodiment according to the present invention for separating a solutionmixture,

FIG. 2 is a diagrammatic view of similar separation system of a secondembodiment of the present invention,

FIG. 3 is a diagrammatic view of a third embodiment of the presentinvention, which is a reactor system,

FIGS. 4A-4C are diagrammatic views of modified examples of the thirdembodiment, and

FIG. 5 schematically shows a separation membrane.

BEST MODE FOR EMBODYING THE INVENTION

The aromatic alkyl compound used in the invention is an alkyl benzenesuch as mono, di or trialkyl benzene, which is to be converted to anaromatic carboxylic acid such as aromatic monocarboxylic acid, aromaticdicarboxylic acid or aromatic tricarboxylic acid by liquid-phaseoxidation, and include alkyl benzenes having their alkyl groupspartially oxidized. The present invention is applicable especially tothe production of a terephthalic acid. The aromatic alkyl compound usedin the invention as the preferable raw material is a paraxylene.

Now description is made on how a terephthalic acid is produced byoxidizing a paraxylene.

Acetic acid as the solvent used in the invention is used in an amount 2to 6 times in weight of the paraxylene, the raw material. This aceticacid solvent may contain a small amount, i.e. not more than 15 percentby weight, of water.

In order to oxidize the paraxylene in a liquid phase, a gas containingmolecular oxygen is used, which is typically air because air can be usedat a low cost with a simple facility. Air may be diluted or enrichedwith oxygen.

In oxidizing the paraxylene, a catalyst containing cobalt (Co),manganese (Mn) and bromine (Br) as its constituent elements is usuallyused.

The paraxylene is oxidized in a liquid phase by continuously supplying agas containing molecular oxygen at 140-230 degrees Celsius, preferably150-210 degrees Celsius in the presence of a catalyst in the acetic acidsolvent. The pressure during the oxidation step has to be at least highenough for the mixture to be capable of maintaining a liquid phase atthe reaction temperature, and is typically in the range of 0.2 to 5 MPa,preferably 1 to 2 MPa.

The reactor is typically a tank having an agitator. But an agitator isnot an essential element. For example, it may be a bubble tower. It hasa port for a molecular oxygen-containing gas at its lower portion.

A molecular oxygen-containing gas that has been supplied into thereactor through its port is used for oxidation, and leaves the reactoras an exhaust gas carrying a large amount of solvent vapor.

The reaction product obtained in the reaction step is formed into areaction slurry, of which the temperature and the pressure is reduced tosuitable levels in a crystallization step to obtain a terephthalic acidslurry. The crystallization is carried out in 1 to 6 steps, preferablyin 2 to 4 steps. In the crystallization step, flush cooling is employed.The final step is preferably carried out in a reduced-pressure, boiledstate.

Typically, the slurry that has been subjected to crystallization issubjected to a solid-liquid separation step and a drying step to recovera terephthalic acid. The terephthalic acid thus recovered may consist ofa low-purity terephthalic acid and a medium-purity terephthalic acid.But the low-purity terephthalic acid may be further refined to ahigh-purity terephthalic acid. In this case, after replacing the aceticacid solvent with an aqueous solvent in a solvent replacement step, thelow-purity terephthalic acid may be directly sent to the refining step,while bypassing the drying step.

In the solvent recovering step, the mother liquid that has beenaliquoted in the solid-liquid separation step, and oxidized exhaust gasor its condensate are refined, typically distilled, to obtain aceticacid.

Using a separation membrane having preference for water, at least partof a mixture of acetic acid, methyl acetate byproducts and reactionwater produced in the oxidation step, solid-liquid separation step andsolvent recovering step is separated into a permeable gas of which themajor component is water and a nonpermeable substances of which aceticacid and methyl acetate are main components to recover methyl acetateand acetic acid.

Separation membranes 8, 46, 84 and 86 (FIGS. 1-3) according to thepresent invention, which have preference for water, are membranes thatpreferentially pass water (H₂O) molecules. That is, when a mixture ofwater and organic compounds is fed through such membranes, water canmore easily permeate them than the organic compound molecules. Morespecifically, these separation membranes have a vapor-acetic acidseparation coefficient α in the range of about 200 to 500 if the vaporconcentration is 20 to 40 weight percent. (The separation coefficient αis expressed by α={(1−Y)/Y}/{(1−X)/X}, where X is the molar fraction ofthe nonpermeative components before permeating the membrane, and Y isthe molar fraction of the nonpermeative components after permeating themembrane.)

Such separation membranes can separate e.g. a mixture of acetic acid andwater into a permeable gas consisting essentially of aqueous substancesof which the major component is water and nonpermeable substances mainlycomprising nonaqueous substances such as acetic acid are maincomponents.

In the preferred embodiment, mixtures fed through separation membranesare gases. Such gases include gases discharged from the oxidationreactor and gases obtained during dehydration/distillation treatment inthe solvent recovering step. Such gases further include gases obtainedby directly supplying exhaust gases from the oxidation reactor directlyinto a distillation column and dehydrating them, and gases obtained bycooling, while releasing pressure, a condensate obtained by condensingat least part of the abovementioned gases.

By feeding such a gas mixture of acetic acid and water through one ofthe separation membranes of the invention, the gas mixture can beseparated into a permeable gas consisting essentially of aqueoussubstances and a nonpermeable gas consisting essentially of nonaqueoussubstances.

If such a gas mixture contains an organic compound or an inert gas inaddition to acetic acid and water, when it permeates one of theseparation membranes of the invention, such an organic compound or inertgas will preferentially remain in the nonpermeable substances, whichconsist essentially of nonaqueous substances, because such an organiccompound or inert gas is less permeable through the membrane.

Any of the separation membranes according to the present invention,which is designated by 111 in FIG. 5, is made of an inorganic material.Specifically, it comprises an inorganic porous member 112 such as aporous ceramic substrate, about 1 mm thick, and a silica gel carryinglayer 113 comprising a silica gel membrane about 10 micrometers thick.The separation membrane may be a flat, tubular or otherwise shapedmember. The silica gel is not limited but is preferably one obtained byhydrolyzing an alkoxysilane containing ethoxy groups or methoxy groupsbecause it improves preference for water.

To such a separation membrane, water (H₂O) is preferentially adsorbed by—OH groups in the silica gel carrying layer 113, thereby inhibitingother components from finding their way into the pores of the silica gelcarrying layer 113. The water adsorbed by the —OH groups moves in thepores and permeates the silica gel carrying layer 113. Thus, the —OHgroups in the silica gel carrying layer 1113 serve to selectivelyseparate and remove water in vapor. As a result, the separation membranereveals preference for water.

Now embodiments of the invention are described. Throughout thespecification, the aqueous component mainly comprising water is referredto as the “first component”, and the nonaqueous component mainlycomprising acetic acid, methyl acetate and the like is referred to asthe “second component”.

EMBODIMENT 1

FIG. 1 shows Embodiment 1 of the present invention, which is aseparation system for separating a solution of a mixture of water andacetic acid.

Typically, the separation system of Embodiment 1 is used to remove waterproduced by oxidation reaction when producing a terephthalic acid byoxidizing, in a liquid phase, a paraxylene as the raw material using airin a reaction solvent containing acetic acid in the presence of anoxidation catalyst.

The separation system of Embodiment 1 includes a distillation column 1having a plurality of fluidized beds such as shelves in the interiorthereof. To an upper portion of the column, 78 weight percent of aliquid-phase aqueous solution of acetic acid (liquid-phase feed) issupplied through an upper supply pipe 2, and to a lower portion of thecolumn, 87 weight percent of an aqueous solution of acetic acid and asmall amount of nitrogen are supplied as a vapor-phase feed through alower supply pipe 3. Thus, a mixture of the first and second componentsis supplied into the distillation column 1.

An overhead vapor supply pipe 4 is connected to the top of thedistillation column 1 such that overhead vapor from the column 1 isintroduced into the supply pipe 4. The pipe 4 branches to a first branchpipe 5 and a second branch pipe 6. In Embodiment 1, overhead vapor isdistributed to the first branch pipe 5 and the second branch pipe 6 inthe ratio of e.g. 9 to 1. A superheater 7 for superheating the overheadvapor is mounted to the downstream end of the first branch pipe 5.Provided further downstream from the superheater 7 is a separator 8including a separation membrane 8 a for separating the overhead vaporinto a permeable vapor mainly comprising steam (first component) andnonpermeable substances mainly comprising acetic acid vapor (secondcomponent).

In this embodiment, the separation membrane 8 a is made of an inorganicmaterial. It passes water or water vapor relatively freely but scarcelypasses acetic acid or acetic acid vapor.

The second branch pipe 6 includes a reflux unit 9 comprising a condenser10 for cooling and liquefying the overhead vapor flowing into the pipe6, a gas-liquid separator 11 for separating the thus cooled overheadvapor into gas and liquid, and a liquid-phase pump 13 for returning theseparated liquid into the distillation column 1 through a return pipe12. Gas separated in the gas-liquid separator 11 is discharged through adischarge pipe 14.

The vapor that has permeated the separation membrane 8 a of theseparator 8 flows into a vapor introducing pipe 15 connected to theseparator 8, and is cooled and liquefied in a condenser 16 provided inthe pipe 16. The thus cooled vapor is separated into gas and liquid in agas-liquid separator 17 connected to the pipe 15. The gas separated inthe separator 17 is discharged through a discharge pipe 18 and a vacuumpump 20 into a gas discharge pipe 21. The liquid separated in theseparator 17 is discharged through a discharge pipe 19 and aliquid-phase pump 22 into a liquid discharge pipe 23.

Nonpermeable vapor that has not permeated the separation membrane 8 a ofthe separator 8 flows into a nonpermeable vapor introducing pipe 24connected to the separator 8, and is cooled and liquefied in a condenser25 provided in the pipe 24. The thus cooled vapor is separated into gasand liquid in a gas-liquid separator 26 connected to the pipe 24. Gasseparated in the separator 26 is discharged through a discharge pipe 27and a pressure valve 28 into the vacuum pump 20 and the gas dischargepipe 21. Liquid separated in the separator 26 is discharged through aliquid-phase pump 29 and a first acetic acid discharge pipe 30.

To the bottom of the distillation column 1, a second acetic aciddischarge pipe 31 is connected through which the bottom layer of theliquid in the distillation column 1, which is high in the acetic acidconcentration, is discharged. Part of the liquid flowing into the pipe31 flows into a circulating pipe 33 extending from an intermediateportion of the pipe 31 to the distillation column 1, is reheated in areboiler 32 provided in the pipe 33, and is returned into thedistillation column 1.

Separation steps performed by the separating system of Embodiment 1 arenow described.

First, the liquid-phase feed A and the vapor-phase feed B are fedthrough the upper supply pipe 2 and lower supply pipe 3, respectively,into the distillation column 1. Since the liquid-phase feed A falls inthe distillation column 1 and the vapor-phase feed B rises in the column1, they will contact each other in the column 1. Part of the liquiddischarged through the second acetic acid discharge pipe 31 is heated bythe reboiler 32 and returned into the distillation column 1 near itsbottom through the circulating pipe 33.

The liquid-phase feed A is a liquid substance of which primarycomponents are water and acetic acid. The vapor-phase feed B is agaseous substance or a liquid substance that is gasified in the column 1and primarily comprises water and acetic acid. The vapor-phase feed Balso includes a substance that remains liquid in the column but isgasified when heated by the reboiler 32 or the reboiler 74 in Embodiment2.

When the feeds A and B are fed into the column, they will be mixedtogether such that water concentration is higher near the top of thedistillation column 1 and the acetic acid concentration is higher nearthe bottom of the column 1.

Overhead vapor (of which the water concentration is relatively high)flows into the overhead vapor introducing pipe 4, and is thendistributed into the first branch pipe 5 and the second branch pipe 6 inthe proportion of 9 to 1.

Overhead vapor that has been introduced into the second branch pipe 6 isreturned into the distillation column 1 by the reflux unit 9. The vaporthus returned into the column 1 further increases the waterconcentration and thus reduces the acetic acid concentration near thetop of the column 1.

Overhead vapor introduced into the first branch pipe 5 is superheated inthe superheater 7 (to prevent the overhead vapor from being liquefiedbefore reaching the separation membrane 8 a), and introduced into theseparator 8.

The thus superheated overhead vapor is separated into a permeable vapormainly comprising water and a nonpermeable vapor mainly comprisingacetic acid.

The reflux unit 9 serves to reduce the acetic acid concentration of theoverhead vapor introduced into the first branch pipe 5 to asubstantially constant value (about 62 weight percent). The acetic acidconcentration of the permeable vapor that has permeated the separationmembrane 8 a thus decreases to less than 1 weight percent.

The permeable vapor is cooled in the condenser 16 and mostly liquefied.After removing nitrogen gas and other gases mixed in the liquid in thegas-liquid separator 17, the liquid is fed by the liquid-phase pump 22and recovered.

The nonpermeable vapor is cooled in the condenser 25 and mostlyliquefied. After removing nitrogen gas and other gases mixed in theliquid in the gas-liquid separator 26, the liquid is fed by theliquid-phase pump 29 and recovered.

Gaseous components removed from the vapor in the gas-liquid separators17 and 26 are sucked by the vacuum pump 20 and discharged from thesystem. The pressure valve 28 prevents the nonpermeable vapor fromflowing toward the gas-liquid separator 17 even if the nonpermeablevapor pressure is higher than the permeable vapor pressure.

The permeable vapor thus produces water containing not more than 1weight percent of acetic acid, while the nonpermeable vapor produces aliquid comprising not less than 93 weight percent of acetic acid. Theliquid produced from the permeable vapor is useful in the plant. Or evenif it is discarded, it will not contaminate the environment because itis practically pure water. The nonpermeable vapor and the liquiddischarged from the lower portion of the distillation column 1 haveenough purity as solvents to be used in the process. When dischargedfrom the top of the distillation column 1, the overhead vapor containsmethyl acetate, which is a byproduct produced during oxidation reaction.It is separated as the nonpermeable vapor together with acetic acid inthe separator 8. The nonpermeable gas, which contains acetic acid andmethyl acetate, is recovered and reused in the oxidation step. Thisreduces the consumption of acetic acid.

In Embodiment 1, the reflux unit 9 is provided to reduce the acetic acidconcentration of the overhead vapor. Thus, separation of the overheadvapor can be carried out so as to meet the requirements of the useraccording to the separation capability of the separation membrane 8 a.The separated liquid needs not be returned into the distillation column1. This makes it possible to use a smaller distillation column 1 andsaves energy.

In Embodiment 1, both the liquid-phase feed and the vapor-phase feed aresupplied into the distillation column 1. But only one of them may besupplied.

In Embodiment 1, a mixed solution is distilled in the distillationcolumn 1. But if it is desired to reduce the size of the entireseparation system, the distillation column 1 may be replaced with anevaporating can.

EMBODIMENT 2

FIG. 2 shows Embodiment 2 of the present invention, which is aseparation system for separating a solution of a mixture of water andacetic acid.

Like the system of Embodiment 1, the system of Embodiment 2 is typicallyused to remove water produced by oxidation reaction when producing aterephthalic acid by oxidizing, in a liquid phase, a paraxylene as theraw material using air in a reaction solvent containing acetic acid inthe presence of an oxidation catalyst.

Like the system of Embodiment 1, the system of Embodiment 2 includes adistillation column 41 having a plurality of fluid beds such as shelvesin the interior thereof. To an upper portion of the column, 78 weightpercent of a liquid-phase aqueous solution of acetic acid (liquid-phasefeed A) is supplied through an upper supply pipe 42, and to a lowerportion of the column, 87 weight percent of an aqueous solution ofacetic acid and a small amount of nitrogen are supplied as a vapor-phasefeed B through a lower supply pipe 43.

To the top of the distillation column 41, an overhead vapor introducingpipe 44 is connected into which overhead vapor spontaneously flows fromthe top of the distillation column 41. To the downstream end of the pipe44, a superheater 45 for superheating the overhead vapor is connected.Downstream from the superheater 45, a first separator 46 is providedwhich includes a first separation membrane 46 a for separating theoverhead vapor into a first permeable vapor mainly comprising steam anda first nonpermeable vapor mainly comprising acetic acid vapor.

The first separation membrane 46 a is identical to the separationmembrane 8 a of Embodiment 1.

The first permeable vapor, which has permeated the first separationmembrane 46 a, is introduced into a first permeable gas introducing pipe47 connected to the first separator 46. The first permeable gasintroducing pipe 47 is provided with a second separator 48 including asecond separation membrane 48 a for separating the first permeable vaporinto a second permeable vapor containing, as its major component, afirst component of the first permeable vapor in a higher concentrationthan the first permeable vapor, and into a second nonpermeable vaporcontaining, as its major component, a second component of the firstpermeable vapor. The second separation membrane 48 a is identical to thefirst separation membrane 46 a. A superheater (not shown) may beprovided between the first separator 46 and the second separator 48 tosuperheat the first permeable vapor.

The first nonpermeable vapor, i.e. the vapor that has not permeated thefirst separation membrane 46 a of the first separator 46, is introducedinto a first nonpermeable vapor introducing pipe 49 connected to thefirst separator 46. The pipe 49 is provided with a condenser 50 forcooling and liquefying the first nonpermeable vapor. The thus liquefiedfirst nonpermeable gas is then separated into gas and liquid in agas-liquid separator 51 connected to the pipe 49. The gas separated inthe separator 51 is introduced into a discharge pipe 52 connected to theseparator 51, while the liquid separated in the separator 51 isintroduced into a discharge pipe 53 connected to the separator 51. Thedischarge pipe 52 is connected through a pressure valve 54 to a vacuumpump 55 and a gas discharge pipe 56. The discharge pipe 53 is connectedthrough a first liquid-phase pump 57 and a second liquid-phase pump 58to a first acetic acid discharge pipe 70.

The second nonpermeable vapor, i.e. the vapor that has not permeated thesecond separation membrane 48 a of the second separator 48, isintroduced into a second nonpermeable vapor introducing pipe 59connected to the second separator 48. The pipe 59 is provided with acondenser 60 for cooling and liquefying the second nonpermeable vapor.The thus cooled and liquefied vapor is separated into gas and liquid ina gas-liquid separator 61 connected to the pipe 59. Gas separated in theseparator 61 flows into a discharge pipe 62 connected to the separator61, while liquid separated in the separator 61 flows into a dischargepipe 63 connected to the separator 61. The discharge pipe 62 isconnected through a pressure valve 64 to the vacuum pump 55. Thedischarge pipe 63 is connected to the second liquid-phase pump 58.

The second permeable vapor, i.e. the vapor that has permeated the secondseparation membrane 48 a of the second separator 48, is introduced intoa second permeable vapor introducing pipe 65 connected to the separator48, and is cooled and liquefied in a condenser 66 provided in the pipe65. The thus cooled and liquefied second permeable vapor is separatedinto gas and liquid in a gas-liquid separator 67 connected to the pipe65. Gas separated in the separator 67 is introduced into the dischargepipe 68 connected to the separator 67, while liquid separated in theseparator 67 flows into a discharge pipe 69 connected to the separator67. The discharge pipe 68 connects to the vacuum pump 55. The dischargepipe 69 connects to a water discharge pipe 72 through a thirdliquid-phase pump 71.

To the bottom of the distillation column 41, a second acetic aciddischarge pipe 73 is connected through which the lowermost layer of theliquid in the distillation column 41, which is high in the acetic acidconcentration, is discharged. Part of the liquid flowing into the pipe73 flows into a circulating pipe 75 extending from an intermediateportion of the pipe 73 to the distillation column 1, is reheated in areboiler 74 provided in the pipe 75, and returned into the distillationcolumn 41.

Now the operation of the system of Embodiment 2 is described.

First, the liquid-phase feed and the vapor-phase feed are fed throughthe upper supply pipe 42 and lower supply pipe 43, respectively, intothe distillation column 41. Since the liquid-phase feed falls in thedistillation column 41, and the vapor-phase feed rises in the column 41,they contact each other in the column 41. Part of the liquid dischargedthrough the second acetic acid discharge pipe 73 is heated by thereboiler 74 and returned into the distillation column 41 near its bottomthrough the circulating pipe 75.

When the feeds are fed into the column, they are distributed in thecolumn such that the water concentration is higher near the top of thedistillation column 41 and the acetic acid concentration is higher nearits bottom.

Overhead vapor (of which the water concentration is relatively high)flows into the overhead vapor introducing pipe 44, is superheated in thesuperheater 45 (to prevent the overhead vapor from being liquefiedbefore reaching the separation membrane 46 a), and is introduced intothe first separator 46.

The thus superheated overhead vapor is separated into a first permeablevapor mainly comprising water and a first nonpermeable vapor mainlycomprising acetic acid.

In Embodiment 2, in order to increase the water concentration of theoverhead vapor to a certain extent, the liquid-phase feed is introducedinto the upper portion of the distillation column 41. Still, the aceticacid concentration of the first permeable vapor is sufficiently high(about 68 percent by weight).

The first permeable vapor is introduced into the second separator 48,and separated into a second permeable vapor mainly comprising water, anda second nonpermeable vapor mainly comprising acetic acid.

Since the acetic acid concentration of the first permeable vapor isabout 5 percent by weight as described above, the acetic acid content ofthe vapor that has permeated the second separation membrane 48 adecreases to less than 1 percent by weight.

The second permeable vapor is cooled in the condenser 66 and mostlyliquefied. After removing nitrogen gas and other gases mixed in theliquid in the gas-liquid separator 67, the liquid is fed by the thirdliquid-phase pump 71 and recovered.

The first separation membrane 46 a of the first separator 46 and thesecond separation membrane 48 a of the second separator 48 are bothcapable of separating water. In order to ensure both high separationspeed and high separation ability, the first separation membrane 46 a ispreferably one which is high in permeating speed, while the secondseparation membrane 48 a is preferably one high in separation ability.

The second nonpermeable vapor is cooled in the condenser 60 and mostlyliquefied. After removing nitrogen gas and other gases mixed in theliquid in the gas-liquid separator 61, the liquid is fed by the secondliquid-phase pump 58 and recovered.

The first nonpermeable vapor, which has not permeated the firstseparator 46, is cooled in the condenser 50 and mostly liquefied. Afterremoving nitrogen gas and other gases mixed in the liquid in thegas-liquid separator 51, the liquid is fed by the first and secondliquid-phase pumps 57 and 58 and recovered.

The second permeable vapor thus produces water containing not more than1 percent by weight of acetic acid, while the first nonpermeable vaporand the second nonpermeable vapor produce a liquid containing 95 percentby weight of acetic acid. A liquid discharged from the bottom of thedistillation column 41 contains 98 percent by weight of acetic acid. Theliquid produced from the second permeable vapor is useful in the plant.Or even if it is discarded, it will not contaminate the environmentbecause it is practically pure water. The first and second nonpermeablevapor and the liquid discharged from the bottom of the distillationcolumn 41 have enough purity as solvents to be used in the process. Theoverhead vapor discharged from the top of the distillation column 41contains methyl acetate, which is produced during oxidation reaction. Itis separated as the nonpermeable vapor together with acetic acid in thefirst and second separators 46 and 48. The nonpermeable vapors, whichcontain acetic acid and methyl acetate, are recovered and reused in theoxidation step. This reduces the consumption of acetic acid.

In Embodiment 2, the first and second separators 46 and 48 are providedto separate the overhead vapor in the first separator 46, and thenre-separate the first permeable gas, i.e. vapor that has permeated thefirst separation membrane 46 a with the second separation membrane 48 ato obtain the second permeable vapor. Thus, the liquid obtained from thesecond permeable vapor has improved purity. The liquids obtained fromthe first nonpermeable vapor, i.e. vapor that has not permeated thefirst separation membrane 46 a, and the second nonpermeable vapor, i.e.vapor that has not permeated the second separation membrane 48 a, alsohave enough purity. Since the liquids produced from these separatedvapors are all high in purity, they do not have to be returned into thedistillation column 41. This makes it possible to use a smallerdistillation column 41 and save energy.

In Embodiment 2, both the liquid-phase feed and the vapor-phase feed aresupplied into the distillation column 41. But only the liquid-phase feedmay be supplied.

In Embodiment 2, a mixed solution is distilled in the distillationcolumn 41. But if it is desired to reduce the size of the entireseparation system, the distillation column 41 may be replaced with anevaporating can.

In Embodiment 2, the first permeable vapor, i.e. the vapor that haspermeated the first separation membrane 46 a, is re-separated with thesecond separation membrane 48 a. But according to the capacity of theseparation membranes, the concentration of the mixed solution and otherconditions, a different arrangement may be employed. For example, thefirst nonpermeable vapor, i.e. the vapor that has not permeated thefirst separation membrane 46 a, may be re-separated with the secondseparation membrane 48 a.

In Embodiment 2, too, the reflux unit 9 of Embodiment 1 may be used.

EMBODIMENT 3

FIG. 3 shows Embodiment 3, which is a reactor system for synthesizingterephthalic acid according to the present invention.

This reactor system includes a reactor 81 filled with an oxidationcatalyst for paraxylene (such as a cobalt compound). To the reactor 81,a raw material supply pipe 82 is connected through which paraxylene asthe raw material, an acetic acid solvent, and an oxidation catalyst aresupplied into the reactor. To the top of the reactor 81, a reactionvapor discharge pipe 83 is connected through which reaction vaporproduced in the reactor is discharged. Air as an oxidant is suppliedinto the reactor 81 through an oxidant supply pipe 101.

To the reaction vapor discharge pipe 83, a first separator 84 includinga first separation membrane 84 a is connected. The first separationmembrane 84 a is identical to the separation membrane 8 a ofEmbodiment 1. Thus, it passes a first component which mainly comprisessteam, but does not pass a second component containing acetic acid vaporand other organic components.

The interior of the reactor 81 is kept at 1-2 MPa and at a temperatureof 100 to 200 degrees Celsius. The vapor supplied through the reactionvapor discharge pipe 83 to the first separation membrane 84 a containssteam, acetic acid vapor and vapor of other organic components, besidesgaseous components derived from the air supplied and gaseous componentsproduced during the reaction.

First permeable vapor, i.e. the vapor that has permeated the firstseparation membrane 84 a of the first separator 84 (of which the majorcomponent is water), is introduced into a first permeable vaporintroducing pipe 85 connected to the separator 84. To the firstpermeable vapor introducing pipe 85, a second separator 86 including asecond separator membrane 86 a is connected. The second separationmembrane 86 a is identical to the first separation membrane 84 a. Asuperheater (not shown) may be provided between the first and secondseparators 84 and 86 to superheat the first permeable vapor. The secondseparator 86 separates the first permeable vapor into a secondnonpermeable vapor comprising a nonaqueous substance of which the majorcomponent is the solvent in the first permeable vapor and a secondpermeable vapor mainly comprising water.

First nonpermeable vapor, i.e. the vapor that has not permeated thefirst separation membrane 84 a of the first separator 84 (of which maincomponents are acetic acid solvent, other organic components and gascomponents derived from the air supplied and reaction gas components) isintroduced into a first nonpermeable vapor introducing pipe 87 connectedto the separator 84. The pipe 87 is provided with a condenser 88 and apressure regulating valve 89.

Second permeable vapor, i.e. the vapor that has permeated the secondseparation membrane 86 a of the second separator 86 (of which the majorcomponent is water) is introduced into a second permeable vaporintroducing pipe 90 connected to the separator 86. The pipe 90 isprovided with a condenser 91 and a liquid-phase pump 92.

Second nonpermeable vapor, i.e. the vapor that has not permeated thesecond separation membrane 86 a of the second separator 86 (of whichmain components are acetic acid solvent, other organic components andgas components derived from the air supplied and reaction gascomponents) is introduced into a second nonpermeable vapor introducingpipe 93 connected to the separator 86. The pipe 93 is provided with acondenser 94 and a pressure regulating valve 95.

The first and second nonpermeable vapor introducing pipes 87 and 93 areconnected to gas-liquid separators 96 a and 96 b, respectively. Gascomponents (such as oxygen, nitrogen, carbon dioxide and carbon monoxidein Embodiment 3) separated in the separators 96 a and 96 b aredischarged through a gas discharge pipe 97 connected to the separators96 a and 96 b, while liquid components (acetic acid solvent and otherorganic components) separated in the separators 96 a and 96 b arereturned into the reactor 81 through a liquid discharge pipe 98connected to the separators 96 a and 96 b. The first and secondnonpermeable vapor introducing pipes 87 and 93, condensers 88 and 94,gas-liquid separators 96 a and 96 b and liquid discharge pipe 98 form areturn passage. A liquid-phase pump 99 may be provided in the returnpassage as shown.

To a lower portion of the reactor 81, a product discharge pipe 100 isconnected through which an acetic acid slurry of terephthalic acidpresent in the form of a liquid in the reactor 81 is discharged.

In Embodiment 3, to the reaction vapor discharge pipe 83, through whichvapor produced in the reactor 81 flows, a distillation unit 121 may beconnected as shown in FIG. 4A. In this arrangement, vapor dischargedfrom the reactor 81 is supplied into the distillation unit 121 anddistilled therein to recover an acetic acid component. Also, gascontaining a smaller amount of acetic acid is discharged from the top ofthe distillation unit 121. The gas thus produced is supplied to thefirst separator 84. The acetic acid component recovered from thedistillation unit 121 is fed into the liquid discharge pipe 98 and thusreturned into the reactor 81. Thus, the distillation unit 121 serves toreduce the acetic acid component in the vapor to be supplied to thefirst separator 84.

In Embodiment 3, to the reaction vapor discharge pipe 83, through whichthe vapor produced in the reactor 81 flows, a condenser 122 may beconnected as shown in FIG. 4B. The condenser 122 serves to condense anycondensable components contained in the vapor discharged from thereactor 81. Any components that are not condensed by the condenser 122are fed into the gas discharge pipe 97.

Any components condensed in the condenser 122 are at least partiallyevaporated in an evaporator 124, and the evaporated components are fedoptionally to the first separator 84 through a heater. Any componentsthat are not evaporated in the evaporator 124 are fed to the gas-liquidseparator 96 a. The condenser 122 and the evaporator 124 make itpossible to treat any noncondensable components contained in the vapordischarged from the reactor 81 while bypassing the first separator 84.

The evaporator 124 is not limited provided it can at least partiallyevaporate the liquid condensed in the condenser 122. For example, if theliquid condensed in the condenser 122 is under pressure, the evaporator124 is preferably a flush tank, a tank which is kept at a lower pressurethan in the condenser 122.

In Embodiment 3, as shown in FIG. 4C, the distillation unit 121 shown inFIG. 4A may be connected to the reaction vapor discharge pipe 83,through which the vapor discharged from the reactor 81 flows, andfurther the condenser 122 shown in FIG. 4B may be connected to adischarge pipe extending from the distillation unit 121. Thisarrangement has both of the functions of the arrangements of FIGS. 4Aand 4B.

Now the operation of Embodiment 3 or the reactor system according to thepresent invention is described.

A paraxylene solution is supplied into the reactor 81 through the rawmaterial supply pipe 82 together with an acetic acid solvent. Air as anoxidant is supplied through the oxidant supply pipe 101. In the reactor81, paraxylene is oxidized by the action of a catalyst, producingterephthalic acid and water. Through the reaction vapor discharge pipe83, a vapor mixture (about 150-200 degrees Celsius) of water, aceticacid solvent, other organic components, gas components derived from thesupplied air, and reaction gas components is discharged.

The vapor mixture discharged from the reactor 81 is introduced into thefirst separator 84 through the reaction vapor discharge pipe 83, and isseparated by the first separation membrane 84 a into the first permeablevapor, of which the major component is water, and the first nonpermeablevapor, of which main components are acetic acid solvent, other organiccomponents, gaseous components derived from the supplied air, andreaction gas components.

In Embodiment 3, as in Embodiment 1, the separation ability of the firstseparation membrane 84 a is such that the first permeable vaporinevitably contains certain amounts of acetic acid solvent, other orgniccompounds, gasesous components derived from the supplied air, andreaction gas components.

However, the first permeable vapor is fed to the second separator 86through the first permeable vapor introducing pipe 85, and separatedagain by the second separation membrane 86 a into the second permeablevapor, of which the major component is water and the second nonpermeablevapor, of which main components are acetic acid solvent, other organiccomponents, gaseous components derived from the supplied air, andreaction gas components. Thus, the second permeable vapor is practicallypure water. The second permeable vapor is introduced into the secondpermeable vapor introducing pipe 90, liquiefied in the condenser 91, fedunder pressure by the liquid-phase pump 92 and recovered in the form ofwater.

The first and second nonpermeable vapors, of which main components areacetic acid solvent, other organic components, gaseous componentsderived from the supplied air, and reaction gas components, areintroduced into the fist and second nonpermeable vapor introducing pipes87 and 93, condensed in the condensers 88 and 94, and fed into thegas-liquid separators 96 a and 96 b, respectively.

The first separation membrane 84 a of the first separator 84 and thesecond separation membrane 86 a of the second separator 86 are bothcapable of separating water. In order to ensure both high separationspeed and separation ability, the first separation membrane 84 a ispreferably one which is high in permeating speed, while the secondseparation membrane 86 a is preferably one which is high in separationability.

In the gas-liquid separators 96 a and 96 b, gas components mixed in thefirst nonpermeable vapor and the second nonpermeable vapor (which mainlycomprise gaseous components derived from the supplied air and reactiongas components) are separated and discharged through the gas dischargepipe 97. Liquid components (which mainly comprise acetic acid solventand other organic components) are discharged through the liquiddischarge pipe 98, optionally pressurized in the liquid-phase pump 99,and returned into the reactor 81 (i.e. to the oxidation step) throughthe raw material supply pipe 82.

Terephthalic acid produced in the reaction during the oxidation step isdischarged through the product discharge pipe in the form of an aceticacid slurry and recovered. It may be refined to obtain high-purityterephthalic acid.

1-7. (canceled)
 8. A method of producing an aromatic carboxylic acidcomprising an oxidation reaction step in which an alkyl aromaticcompound is subjected to liquid-phase oxidation reaction using anoxygen-containing gas in a solvent containing acetic acid in thepresence of an oxidation catalyst to produce a slurry of said aromaticcarboxylic acid; a solid-liquid separation step in which said slurry isseparated into a reaction mother liquid and an aromatic carboxylic acidcake; and a step of separating at least a portion of a mixture of aceticacid and water produced during said steps into a permeable gas mainlycomprising water and nonpermeable substances mainly comprising aceticacid by use of a separation membrane having selectivity for water. 9.The method of producing an aromatic carboxylic acid of claim 8 whereinat least a portion of the mixture fed to said separation membrane is agas.
 10. The method of producing an aromatic carboxylic acid of claim 8wherein said mixture of acetic acid and water further contains methylacetate, and wherein using said separation membrane having selectivityfor water, at least a portion of said mixture is separated into saidpermeable gas, which mainly comprises water, and said nonpermeablesubstances, which mainly comprises acetic acid and further containingmethyl acetate as another main component.
 11. The method of producing anaromatic carboxylic acid of claim 10 wherein said mixture is produced insaid oxidation reaction step, wherein using said separation membranehaving selectivity for water, at least a portion of said mixture isseparated into said permeable gas, which mainly comprises water, andsaid nonpermeable substances, which mainly comprises acetic acid andmethyl acetate, and wherein said nonpermeable substances are at leastpartially returned to said oxidation reaction step.
 12. The method ofproducing an aromatic carboxylic acid of claim 10 wherein at least aportion of a mix of acetic acid, a methyl acetate as a byproduct, andwater, said mix being produced in a production process, is supplied intoa distillation column, wherein at least a portion of the acetate in saidmix is recovered from a bottom of said distillation column, wherein atleast a portion of said mix is produced from a top of said distillationcolumn as said mixture containing acetic acid, methyl acetate and water,wherein using said separation membrane having selectivity for water, atleast a portion of said mixture is separated into said permeable gas,which mainly comprises water, and said nonpermeable substances, whichmainly comprises acetic acid and methyl acetate.
 13. The method ofproducing an aromatic carboxylic acid of claim 12 wherein a portion ofsaid mixture produced from the top of said distillation column isreturned to said distillation column, and the remainder of said mixtureis separated, using said separation membrane having selectivity forwater, into said permeable gas, which mainly comprises water, and saidnonpermeable substances, which mainly comprises acetic acid and methylacetate.
 14. The method of producing an aromatic carboxylic acid ofclaim 12 wherein said nonpermeable substances are returned to saidoxidation reaction step.
 15. The method of producing an aromaticcarboxylic acid of claim 8 wherein using a separation membrane havingselectivity for water, said permeable gas, which mainly comprises water,is further separated into a permeable gas mainly comprising water andnonpermeable substances mainly comprising acetic acid.
 16. The method ofproducing an aromatic carboxylic acid of any of claim 15, wherein one ofsaid separation membranes that is provided upstream from the other isone that is higher in the permeating speed, and the other is one that ishigher in the separation ability.
 17. The method of producing anaromatic carboxylic acid of claim 8 wherein said separation membrane orsaid separation membranes are made of an inorganic material.
 18. Themethod of producing an aromatic carboxylic acid of claim 17 wherein saidseparation membrane or said separation membranes comprise an inorganicporous member carrying in pores thereof a silica gel obtained byhydrolyzing an alkoxysilane containing ethoxy groups or methoxy groups.19. The method of producing an aromatic carboxylic acid of claim 8wherein said alkyl aromatic compound is paraxylene, and said aromaticcarboxylic acid is terephthalic acid.